This manga makes a mistake a lot of dragon ball fans who mock Yamcha refuse to consider.

Yamcha's power limit is achieved when he trained with kaoi-sama, so unlike gohan who got his potential unlocked at Namek but got stronger later on also growing up, Yamcha's power limit should have reached that point in this manga.

The power limit Yamcha achieved was big enough to defeat Rikun 1 on 1 with no real effort. Rikun's power level was highenough to defeat vegeta easily even if he had already powered up from recovering from the battle at earth. So Yamcha becomes strong enough to have defeated vegeta at the earth battle

My guess? Effort. Dragongarowlee puts a bunch of effort into his art, and as a result he can only do so much. I'm pretty sure the magazine releases were delayed because he needed more time. He couldn't do a monthly manga like Super.